(1) Field of the Invention
This invention relates to a lithographic printing plate fabricated by writing information on a highly sensitive photoconductive material for a short period of time, and said photoconductive material involving a binder resin into which a phthalocyanine pigment, zinc oxide and zink sulfide are dispersed.
(2) Description of the Prior Art
With rationalization of office work, word processors have recently spread. In this connection, a conventional typewriter types out characters on papers one by one, whereas a word processor stores once inputted characters in the memory as electrical signals and at the same time, reads out such electrical signals to display them in the cathode-ray tube as the characters.
On the other hand, in order to fabricate a printing plate on the basis of the electrical signals which had been stored in the memory of a word processor, characters have been once outputted on papers as the hard copy in accordance with ink jet printing, heat sensitive recording, wire-dot recording, electrophotographic or the like method, and a zinc oxide master recording paper is charged, exposed and fixed by using the hard copy as an original material, whereby the printing plate is obtained. However, such process as described above requires much work and time because there are two steps of outputting for the hard copy and of plate making. The process has also such disadvantage that quality of the image in a printing plate deteriorates since the image must be copies on the printing plate from the hard copy. Furthermore there is a process in which image is directly exposed on a photoconductive plate by means of laser light to form latent image, and then the latent image is developed and fixed, whereby a printing plate is fabricated. In this process, argon, helium-neon, helium-cadmium YAG, carbonic acid gas or the like laser is utilized. There are, however, such disadvantages that these lasers become oversized in order to obtain a required output, and power consumption thereof is too remarkable for the amount of light outputted. In this respect, comparing semiconductor laser with those described above, the semiconductor laser has such characteristic features of favourable microminiaturization, high efficiency, low voltage, low power consumption, being capable of effecting high speed modulation over 1 GHz by means of driving electric power, high reliability peculiar to semiconductor and the like. However, semiconductor laser, such as AlGaAs laser light of a semiconductor laser which is put to practical use is 760.about.800 nm or 820.about.880 nm, has a small amount of outputted light and a wavelength outputted is longer in comparison with gas laser light. In this connection, there is such a fatal disadvantage that zinc oxide master paper which has heretofore been used in this technical field has no sensitivity with respect to long wavelength laser light. As photoconductive materials having sensitivity with respect to such long wavelength, CdS-Cu, Se-Te/Se, amorphous silicon, phthalocyanine pigments and the like are known, but only the phthalocyanine pigments can be put to practical use in view of toxicity and cost. However, since phthalocyanine pigments have such small average particle diameter of 0.05.mu. and are electrically low resistant, it is required that a ratio of components of a binder resin makes higher in order to utilize any phthalocyanine pigment in an electrophotographic plate prepared by dispersing such phthalocyanine pigment into the binder resin. In this connection, when prepared an electrophotographic plate from only a phthalocyanine pigment and a binder resin, the surface of the resulting electrophotographic plate becomes very smooth so that fixation of toner is very unfavourable. For this reason, there is such a disadvantage that even though a lithographic printing plate was fabricated from the electrophotographic plate, such toner separates from the electrophotographic plate during printing operation and accordingly, such electrophotographic plate could not have been utilized.
The present invention contemplates to eliminate disadvantages of said phthalocyanine electrophotographic plate and for which tests were conducted by mixing fillers which mat the surface of electrophotographic plate with a binder resin and a phthalocyanine pigment for improving fixation of toner and hydrophilic property of the electrophotographic plate when employed as a lithographic printing plate. As a result of the tests through selecting titanium oxide, calcium carbonate, cadmium carbonate, magnesium oxide, zinc oxide, or zinc sulfide as a filler, it was found that while matted surface of the resulting electrophotographic plate was attained, photosensitivity fell much more than, dark decay became larger than, and charge retentivity became also inferior to those of an electrophotographic plate composed of phthalocyanine pigment-resin. On one hand, it was also found that an electrophotographic plate prepared by dispersing phthalocyanine pigment and zinc oxide and zinc sulfide into a binder resin could form minute unevenness on the surface of the resulting photoconductive layer by means of zinc oxide and zinc sulfide without decreasing photosensitivity thereof, because of a larger average particle diameter of 0.3.mu. in zinc oxide and zinc sulfide than that of phthalocyanine pigment, of a high value of electric resistance of said electrophotographic plate, and of such fact that a ratio of components in the binder resin can be reduced in case of a phthalocyanine pigment alone, and thus zinc oxide and zinc sulfide were desirable for the fillers to be contained in the electrophotographic plate used in the present invention.
In phthalocyanine pigment-resin electrophotographic plates, since the phthalocyanine pigment pertains principally to photosensitivity, electrophotographic characteristics in positive charge are favourable. However, the present inventors have obtained such novel knowledge that an electrophotographic plate to a photoconductive layer of which has been added only zinc oxide as the filler has poor charge retentivity and poor dark decay property with respect to positive charge, whilst an electrophotographic plate comprising only zinc sulfide as the filler has such disadvantage that although there are good charge retentivity and good dark decay property with respect to positive charge, residual potential becomes remarkable.